This invention relates to an apparatus and method for purifying gases and, in particular, to an apparatus and method for purifying hydrogen gas.
Many present day industries utilize in their manufacturing processes hydrogen of over 99% purity. Chemical producers, food manufacturers, and electronics manufacturers, to name a few, require pure hydrogen for various purposes.
Most industrial processes for producing hydrogen provide the hydrogen in less than 75% concentration. Catalytic steam reforming of natural gas or light hydrocarbons, partial oxidation of heavy hydrocarbon stock and coal gasification all produce dilute hydrogen. Also, dilute hydrogen is often available as a by-product in several chemical industries.
To obtain pure hydrogen, small scale users generally employ a costly electrolytic process or purchase merchant hydrogen. Large scale users, on the other hand, use state-of-the-art purification techniques such as scrubbing, cryogenic separation, pressure-swing adsorption, or membrane diffusion separators.
In the conventional electrolytic type of processing, hydrogen contained in water is converted to gaseous hydrogen by oxidizing water in an electrochemical cell. In another electrolytic process, the hydrogen contained in a gaseous mixture is transformed into ionic form in contact with a palladium membrane. The ionic hydrogen at the surface of the membrane is converted to atomic hydrogen and the atomic hydrogen then passes through the membrane. At the output end, the atomic hydrogen is converted to molecular hydrogen and thereby pure hydrogen is produced.
The above method, however, is disadvantageous in that it is sensitive to temperature and impurities such as sulfur and hydrocarbon compounds. Also, the pressure of the hydrogen on the input side of the membrane must always be higher than that on the output side. The palladium based membranes are also prone to loss of stability after repeated cycles of adsorption and desorption.
U.S. Pat. No. 3,446,674 to Giner discloses an electrochemical converter which likewise relies on atomic hydrogen being generated and being passed through a palladium membrane. More specifically, Giner discloses a converter which employs an anode which is provided with a dehydrogenation catalyst. The cathode member is a palladium membrane permeable to hydrogen. An electrolyte is disposed between the anode and cathode and a power supply is connected to the anode and cathode to complete the circuit.
In operation of this converter, a gaseous mixture such as hydrocarbon and steam is passed into contact with the anode and undergoes a reaction under the influence of the dehydrogenation catalyst and current to produce hydrogen ions and carbon dioxide. The hydrogen ions pass from the catalyst through the electrolyte to the cathode palladium membrane where they accept electrons to form atomic hydrogen. The atomic hydrogen then permeates through the membrane and in the manifold at the outlet side of the membrane is formed into molecular hydrogen which is now substantially pure.
Giner also discloses that the anode of his converter may be formed by coating a conductive metal screen with a suitable dehydrogenation catalyst and treating the screen with a hydrophobic material. In this regard, Giner states that other structures may be employed for fabricating the anode, including metal elements inherently permeable to gases such as the porous electrode structure disclosed in Bacon U.S. Pat. No. 2,928,783 and that the metal may be inherently catalytic such as palladium and platinum.
The converter described by Giner relies upon the permeation of atomic hydrogen through a palladium cathode membrane in order to effect hydrogen purification. This makes the cell sensitive to temperature variations and impurity levels, as well as requiring a large differential pressure across the palladium membrane for operation. Furthermore, Giner acknowledges that part of the hydrogen will be evolved on the cathode face of the palladium membrane. This will result in hydrogen loss and seal leaks. Also, the palladium membrane has a high hydrogen over voltage which makes it prone to large power consumption. Finally, the stability of the membrane in an acid media and under the required operating temperature and repeated cycling is also questionable. The Giner converter thus is disadvantageous in a number of respects.
It is therefore an object of the present invention to provide an apparatus and method for purifying hydrogen which do not suffer from the above disadvantages.
It is a further object of the present invention to provide a hydrogen purification apparatus and method which can provide an output gas pressure which is approximately equal to or greater than the input gas pressure.
It is yet a further object of the present invention to provide a hydrogen purification apparatus and method which are highly stable and which provide increased output capacity.